Fix a couple of bugs causing Jail.Exec not to work.

 * The jail ID should be the first parameter passed to `jexec`.
 * The modified arguments, not the originals, should be passed to
   `jexec`.
Add in VnodeBacked/MallocBacked methods to MDDev.
Change Jail.Exec to return the exec.Command without executing it.

BREAKING API CHANGE.

This allows the calling code to set any additional fields in the Command
struct (e.g., for setting stdin/stdout or changing the env).